1. Field of the Invention
The present invention relates generally to automatic focusing control, and more particularly, to a method and apparatus for auto-focusing in an image sensor that accounts for a bright light source in an image.
2. Background of the Invention
Automatic focusing is now essential within image sensing systems such as digital cameras and mobile phones with a digital camera. In addition, the performance of an image sensing system is indicated by the accuracy and speed for driving a focusing or taking lens to an in-focus position in various environments.
For automatic focusing (i.e., auto-focusing), characteristics such as sharpness of an image captured by an image sensor are analyzed, and a focusing lens is moved to a position (e.g., an in-focus position) resulting in a maximum focus value. A focus value curve is used such that a clearest (i.e., in-focus) image is captured. The image sharpness information is high-frequency image signal information having a maximum value when the image is in-focus.
Generally, an image sensing system with auto-focusing determines a focus value from an image captured by an image sensor and analyzes the focus value for controlling the position of a focusing lens. Accordingly, accurate detection of the focus value is desired for in turn accurately controlling the position of the focusing lens.
However, accurate detection of a focus value is difficult for an image having low-contrast detail and saturated highlight area (e.g., an image including a light source) in a conventional image sensing system like a digital camera. In other words, the image sharpness information used to determine the focus value is significantly influenced by image noise and a saturated highlight area (e.g., a light source) included in the captured image.
Even when an image is generated with a light source, e.g., a glow lamp, a fluorescent lamp, or the sun being outside the in-focus image, the image of the light source includes strong edges which distort the image sharpness information. Accordingly, setting the focusing lens to the in-focus position is difficult for such an image including a light source in the conventional image sensing system.